Climatic Drivers Update

Weather Matters climate update - Rainfall for May 2021.

% chance of exceeding median rainfall for the month of May.

Weather Matters climate update - Temperature Outlook for May 2021

%chance of exceeding the monthly median temperatures for May 2021.


Indian Ocean - Warm neutral - little influence overall.

The Indian Ocean has very little influence on rainfall during the month of May and into June, however the influence does increase mid winter through spring.

A positive IOD equals less rainfall and high temperatures for much of southern and eastern Australia.

A negative IOD equals more rainfall and lower temperatures for much of southern and eastern Australia.

Neutral phase = average conditions generally*

Tropics - MJO transitioning out of the southern hemisphere - no influence.

The MJO is currently over the Americas bringing widespread rainfall and cloudiness out over the region. When it is placed there, it has a suppression impact on rainfall over the Maritime continent. This time of year the MJO has little impact as it is moving away from Australia and towards the equator.

Southern Annular Mode - Neutral - little influence overall.

The SAM is expected to remain relatively neutral for the coming 2 weeks and likely 3 weeks before turning negative by the time we get to around May 21st. From that time I do expect a larger impact of frontal weather as the phase shifts out of neutral to that negative phase.

Positive phase of the SAM usually means more risk of rainfall over eastern QLD and NSW with easterly winds developing and frontal weather further south of the nation, with little impact during winter.

Negative phase of the SAM usually means more risk of frontal activity impacting southern Australia with increased rainfall odds. Less rainfall and drier westerly winds and above average temperatures are usually observed through northern NSW and coastal NSW north of Sydney and through much of QLD.

ENSO - Neutral - little influence overall.

The Pacific has come out of the La Nina and is recovered to neutral values. More cloudiness over the dateline and rainfall increasing through the region and a relaxation of trade winds supports this occurring.

The Pacific ocean should remain neutral to cool neutral into spring.

Some signals from climate agencies that another weak La Nina may form during our Summer but I think keeping it neutral is the appropriate forecast for now.

Little influence is expected from the Pacific during the next 6 months.

With the impending shift to a winter time westerly, wet weather is expected to develop for the southwest of the nation, spreading through remaining coastal areas during May.

The increasing factor on rainfall being above average through the SWLD of WA and parts of SA is connected to the residual tropical moisture sitting off the NW of the nation over the Indian Ocean, which is likely to be drawn down into the jet stream and lifted ahead of cold fronts. We will see the first of a few events such as this take place, early next week.

GFS Precipitable Water values showing the moisture being drawn into SW WA and possibly into SA during the first half of May.

Chances of exceeding the monthly rainfall for the month of May for remaining southern and eastern parts of the nation are relatively neutral on current guidance. Coastal areas may see a better chance through Victoria and Tasmania, connected to expected frontal weather.

Inland parts of NSW and southern QLD, again a 50% chance of exceeding the rainfall for the month of May, with the chances connected to rain events spreading through the jet stream, if these fail to occur, then a dry month will be likely.

Rainfall connected to the easterly winds will be decreasing as we go through the month, with the westerly winds developing further south. The coast could see a wet start to the month which may increase the chances of exceeding monthly rainfall for May, but there will be longer breaks in the rainfall.

A drier trend is developing over northern Australia, that is in line with the dry season developing, which is nothing to get too concerned about. Very normal.

Drier air coming into the region during the first 10 days of May 2021. Euro 12z run 26th of April 2021.

Overall with no major drivers in play - there are no current signals away from that tropical moisture coming into WA early in the month, to suggest widespread heavy rainfall for the coming month.


Cooler weather is likely over the southwest of the nation, while the north is expected to remain hot and humid for the first week or two of May.

That will lead to a warmer than average month of weather for the region over Northern Australia. But that doesn't mean that the dry season won't come in during this time, and I do think day time temperatures will start to reduce and minimums return to normal or below average as that drier air comes in mid to late month.

Seeing all that red up over northern Australia does not mean that it will not cool down through May, but it will be hot to start the month.

Cooler than average weather is expected for large parts of southwest WA with frontal weather and large amounts of residual tropical moisture leading to cloudy conditions and rain periods for most of the first week of May.

GFS Precipitable water anomalies and MSLP for the next 2 weeks. Valid April 25th 2021.

Euro Temperature anomalies for May 2021

Again, like the north, there will be periods of brilliant sunshine and warmer weather, but with further frontal weather and cooler rainy skies returning mid to late month, the overall month is more likely to be cooler than normal.

Elsewhere, the month of May looks average.

With climatic drivers generally in neutral phases, there is no great pull to it being anything other than average. That also corresponds to rainfall odds too.

Next update due - Friday May 7th for the two week outlook

June Outlook - Available Friday May 14th 2021.

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